The invention concerns a novel method and apparatus for the removal of hydrocarbons from a compressed air/gas stream. Specifically, the invention utilizes a catalytic oxidation method and system to remove hydrocarbons from compressed air.
Compressed air and/or gas has a wide variety of industrial uses. For example, compressed air or gas may be utilized to transmit power, such as in a system for operating pneumatic tools. Alternatively, compressors are often utilized to provide air for combustion in various apparatus. Compressed air may also be utilized to transport and distribute material, such as in air conveying. Compressed air is also used in instrumentation systems throughout industry. Other uses for compressed air or gas include producing or creating conditions more conducive to certain chemical reactions or processes, as well as producing and maintaining desired pressure levels for many purposes. Such uses may require the removal of contaminants which have either leaked or flowed into the system, or are initially present, although unwanted.
Generally speaking, compressed air or gas may be generated by either oil-lubricated type compressors (hereinafter "lubricated compressor(s)") or oil-free, non-lubricated type compressors. The latter oil-free or non-lubricated type compressors are relatively expensive to manufacture, operate and maintain. In this regard, the initial cost of purchasing or providing such a non-lubricated compressor is higher than a comparable oil lubricated type compressor. Moreover, non-lubricated compressors generally consume significantly greater quantities of energy in operation, being typically from 3 percent to 15 percent less efficient than lubricated compressors. Hence, in general, non-lubricated compressors cost 15 percent to 100 percent more than lubricated compressors to purchase initially and require about 25 percent to 50 percent more maintenance. Heretofore, many industrial users have been willing to absorb these higher costs because of problems caused by compressor lubricants in the less expensive lubricated type of compressors.
In this latter regard, while oil-lubricated compressors are more energy efficient, cost significantly less to purchase and require less maintenance than non-lubricated compressors, lubricating oil carry-over (hydrocarbons) in the downstream compressed air causes a number of problems in practice.
Due to the relatively high temperatures and pressures utilized in the air compression process, the lubricants in the downstream air undergo several changes. The oils have in effect been fractionated and cracked and have lost, or been greatly reduced in, their lubricating properties. These oils or hydrocarbons often further mix with water and/or solid particulate matter or "dirt" present in the air/gas stream which may cause severe damage to downstream components. Such problems may include washing away of lubricants required on the downstream instruments or machinery resulting in increased wear and increased required maintenance thereof. This combination of oil, dirt and water in the downstream air can also cause automatic valves, cylinders and like equipment to operate either slowly, unreliably or not at all, as well as causing malfunctions of instrumentation in the air/gas stream. In some systems, product spoilage is caused by these unwanted contaminants, and excessive rust and/or abrasion of downstream parts or products may occur. It has also been found that these contaminants in the air/gas stream can cause outdoor air lines to freeze in cold weather.
Additionally, in an air compressor system, oxygen is always present, and where petroleum oils are used as a lubricant, there must be some concern for the potential of fire or explosion in the system. A source of energy for ignition may be provided by friction, static electricity or heat from the compressor, often in the form of hot carbon particles in the air/gas stream. Most commonly, the petroleum oils used as a lubricant, and present to some degree in the air/gas stream, decompose to form such carbon particles. These particles form deposits which tend to collect on the valves, heads, discharge ports, and in piping in delivery and utilization systems. Tests have shown that such carbon deposits absorb oxygen from the air and under certain conditions generate heat. This heat may reach a point where ignition occurs in the carbon deposits, and such ignition may cause further fire or explosion elsewhere in the system, as well.
Moreover, many treatment systems include drying devices for removing moisture from the compressed air. These drying devices generally work by heating the compressed air, which is often initially at a relatively elevated temperature from compression. In the presence of such relatively elevated temperatures and heating devices, the presence of hydrocarbons in the system can pose a danger of fire or explosion. That is, the hydrocarbon based compressor lubricant lost through bypass or thermal cracking is often transmitted into the compressed air, and the resulting hydrocarbons contaminate treatment and/or distribution systems downstream, often becoming trapped in treatment sections where they sometimes ignite or detonate. In this regard, conventional treatment sections often include mechanical oil filtering devices. While oil and cracked oil products are always present as low concentration contaminants in lubricated systems, concentration can rise over long periods of operation to a point where serious problems are caused or threatened. Such problems are particularly acute for drying systems which operate at elevated temperatures and therefore can more easily cause ignition in the presence of excess hydrocarbons.
A number of lubricants are utilized in air compressors, refined petroleum products being the most prevalent. Synthetic type lubricants are also utilized, and these latter materials are believed to provide a lesser danger of fire or explosion in a system. However, volatile pyrolysis products are often produced for such synthetics in the system which can still cause a danger of fire or explosion. Synthetic lubricants have other disadvantages as well. Due to the energy intensive manufacturing processes utilized in their production, synthetic lubricants are from five to seven times as expensive as petroleum based lubricants. Additionally, most synthetic lubricants tend to exhibit relatively low viscosity, causing low temperature handling problems. Moreover, many commonly used gasket, seal packing and lubricator materials are attacked by synthetic lubricants.
In addition to the foregoing, one particularly advantageous type of dryer, known as a regenerative heat of compression drying system generally cannot be utilized with lubricated compressors. Such heat of compression drying systems generally reuse the heat energy generated during the compression process which is otherwise lost as waste heat energy. Hence, such heat of compression type dryers are relatively inexpensive to operate. Since they utilize a source of energy already present in the system, such dryers virtually eliminate the conventional energy costs of drying air. However, these energy efficient heat of compression drying systems operate at elevated temperatures such that they are normally ruled out for use in connection with lubricated type compressors. That is, because of the presence of hydrocarbons in the downstream flow from such lubricated compressors, the relatively high temperature of operation of heat of compression drying systems is generally believed to pose too great a threat of auto ignition to justify their use.
While, as mentioned above, mechanical filters have been utilized in an effort to remove hydrocarbons from the compressed air stream from lubricated compressors, such mechanical filtering is of limited usefulness. For example, a typical filter operates in liquid phase and hence can only remove hydrocarbons in liquid phase from the air stream. However, since the air is generally at an elevated temperature leaving the compressor and can approach saturation with oil vapor, hydrocarbons or oil products still in vapor form will pass through the filter, and as the air cools downstream of the filter, will condense into the liquid state.
Moreover, conventional filters require daily draining and periodic replacement of filter cartridges, in the absence of which they rapidly become ineffective. Such maintenance procedures are of course relatively time consuming and expensive and can require system shutdown to carry out. However, as mentioned above, even properly maintained filters can collect a quantity of hydrocarbons over a period of operation, thus posing potential ignition or detonation dangers in the presence of the elevated temperatures of the compressed air and gases in the system.
The development of hydrocarbon catalysis coincides with the arrival of the petroleum age, when natural oil and gas provide most of our energy and an increasing share of raw materials for chemical industry. According to the well known principles of catalytic action, unstable species may result when a hydrocarbon molecule collides with the active center of a catalyst. The nature and reactivity of these intermediates determine the products of catalysis and the rate of reaction.
Vapor phase catalytic oxidation and reduction is used for the removal of a large variety of objectionable compounds from many types of gas streams. Catalytic oxidation is particularly suitable for removing small amounts of combustable contaminants from gas streams containing these compounds in concentrations below the flammable limit, and, therefore, has found wide application in the field of air pollution and odor control. Similar applications include automobile exhaust catalytic converters and carbon monoxide converters for breathing air in industrial compressed air systems. The catalytic oxidation apparatus and system for removal of hydrocarbons from compressed air/gas differs from these similar systems in several significant ways. Automotive exhaust systems are designed to remove trace amounts of hydrocarbons from atmospheric pressure emmission systems operated at extremely high temperatures for the protection of the environment. Carbon monoxide converters are used in industrial applications for the conversion of carbon monoxide, a potentially deadly contaminant, into harmless carbon dioxide when used in breathing air apparatus.
None of the catalysts utilized in these related applications would perform when tested in the catalytic oxidation system for removal of hydrocarbons in compressed air. The basic design considerations of the catalytic oxidation system preclude utilization of existing technology and in fact required the development of new catalysts and technology.